Electric motor with permanent-magnet rotor having viscous shaft coupling

ABSTRACT

An electric motor with a permanent-magnet rotor having a viscous coupling to the shaft. In its electric part, the motor can be likened to a permanent-magnet synchronous motor, in which the rotor is mounted freely on the shaft and is contained in a hermetic casing which is rigidly coupled to the shaft and contains a liquid which fills the gap between the rotor and the container. The rotor therefore drives the casing only by virtue of the presence of the interposed liquid, so that the load is not rigidly connected to the shaft and the resulting operation of the motor can be likened to that of an asynchronous motor allowing mutually different rotation rates of the shaft and of the rotating field.

TECHNICAL FIELD

The present invention relates to an electric motor with permanent-magnetrotor having viscous shaft coupling.

BACKGROUND ART

Conventional electric motors having a permanent-magnet rotor comprise astator, with an electromagnet constituted by a lamination pack and bycorresponding windings, and a rotor, which is arranged between two polesformed by the stator and is axially crossed by a shaft which isrotatably coupled to a supporting structure.

It is also well-known that the higher the inertia of the load applied toa synchronous motor, the more difficult it is to start the motor.

Starting in fact occurs as a transient process in which the rotationdirection, the speed and the current change until the synchronous stateis reached.

During this transient process, the rotor oscillates due to thealternating magnetic field produced by the stator, which by inducing atorque on the permanent-magnet rotor tends to move the rotor into aposition in which the magnetic field of the rotor is aligned with thestator field.

If, during this oscillation, the rotor acquires enough kinetic energy tomove imperceptibly away from the alignment position, it undergoes afurther acceleration which makes it perform another portion of a turnand so forth until the synchronous state is reached.

For an equal power level, the lower the inertia of the applied load, thegreater the extent of the oscillations produced on the rotor;accordingly, the rotor is able to accelerate, gaining a speed whichallows it to synchronize with the alternating field of the stator.

Viceversa, if the inertia of the load is significant, the extent of theoscillation of the rotor is limited and does not allow to reach thesynchronous state.

If the inertia of the load is even greater, the extreme case occurs inwhich once power has been supplied to the stator the rotor cannot evenstart the oscillation, i.e., it remains motionless in its equilibriumposition.

For load inertias which are not too high with respect to the power levelof the motor, couplings of the mechanical type are currently widely usedwhich are inserted between the load and the rotor and allow the rotor,during startup, to oscillate freely through a certain rotation angle(usually 180 sexagesimal degrees).

In this manner, in the startup transient the rotor is disengaged fromthe inertia of the load and this is advantageous for attaining thesynchronous state.

Accordingly, a free rotation occurs through a certain angle, followed bya sudden impact when the load is engaged.

At this point a direct connection between the load and the rotor isobtained; in practice, in operation the two are rigidly coupled.

The mechanical couplings are disclosed in EP 723329, where reference isalso made to the application of the motor for a drain pump for a washingmachine or dishwasher.

Usually, the inertia represented by the impeller of a pump for such anapplication is relatively low with respect to the power that can besupplied by the motor. Accordingly, these couplings fully achieve theirfunction, which is indeed to reduce the torque required for startup,giving the correct power rating to the motor with respect to the loadthat it must drive, providing a consequent benefit to the overallefficiency of the machine and therefore to the cost.

However, there are applications in which the inertia of the load (forexample the impeller of a fan) is so great that even the above-citedmechanical coupling is able to start it, unless the motor is oversizedso much that it is excessively expensive to manufacture and use, makingit accordingly uninteresting for the user.

For these applications, the solution is to provide a system which isable to transmit the torque of the motor gradually to the load duringstartup.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to provide an electric motor with apermanent-magnet rotor in which gradual traction occurs at startup andin which the rotation rate of the load can be independent of therotation rate of the rotor.

Within the scope of this aim, a consequent primary object of the presentinvention is to have a low static torque required to start thepermanent-magnet synchronous motor.

Another important object of the present invention is to provide a motorwhich is constructively simple and compact.

Another important object of the present invention is to provide a motorwhich is quiet at startup and during operation.

Another object of the present invention is to provide a motor havingreduced consumption and a low cost.

This aim, these objects and others which will become apparenthereinafter are achieved by an electric motor with permanent-magnetrotor comprising a stator, with an electromagnet constituted by alamination pack and associated windings, and a rotor, which is arrangedbetween two poles formed by the stator and is axially crossed by a shaftwhich is rotatably connected to a supporting structure, characterized inthat said rotor is mounted freely on the rotation shaft to which theload is applied and is contained in a hermetic casing which is rigidlycoupled to said shaft and contains a working fluid, said rotor and saidouter casing being shaped so as to mutually interact only by means ofthe working fluid, thus allowing smooth variations between the speed ofthe rotor and the speed of the casing and accordingly between the rotorand the applied load.

Advantageously, the space between the outer surface of the rotor and theinner surface of the casing contains a viscous liquid, so that therotor, by moving said liquid, turns the casing and therefore the shaftwith the load applied thereto.

Conveniently, in a conceptually equivalent different embodiment a bladedimpeller is rigidly coupled to at least one of the ends of the rotor andinteracts with a corresponding bladed impeller which is rigidly coupledto said casing and is arranged frontally thereto, so as to provide aviscous actuation coupling between the rotor and the casing.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the electric motor accordingto the present invention will become apparent from the followingdetailed description of some embodiments thereof, illustrated only byway of non-limitative example in the accompanying drawings, wherein:

FIG. 1 is a longitudinal sectional view of a motor according to theinvention in a first embodiment;

FIG. 2 is a sectional view, taken along the plane II—II of FIG. 1;

FIG. 3 is an exploded view of the components of the motor of FIG. 1;

FIG. 4 is a longitudinal sectional view of a motor in a secondembodiment thereof;

FIG. 5 is a longitudinal sectional view of a motor in a third embodimentthereof;

FIG. 6 is a longitudinal sectional view of a motor in a fourthembodiment thereof;

FIGS. 7 and 8 are exploded perspective views of the components of themotor of FIG. 6;

FIG. 9 is a longitudinal sectional view of a motor in a fifthembodiment;

FIG. 10 is a sectional exploded view of some of the components of themotor of FIG. 9;

FIG. 11 is a sectional view, taken along the plane XI—XI of FIG. 9;

FIG. 12 is a perspective view of the rotor of FIG. 9;

FIG. 13 is a longitudinal sectional view of a motor in a sixthembodiment;

FIG. 14 is a sectional exploded view of the components of the motor ofFIG. 13.

WAYS OF CARRYING OUT THE INVENTION

With reference to the above FIGS. 1 to 3, in a first embodiment asynchronous permanent-magnet motor comprises a stator 10, constituted bya lamination pack 11 and by windings 12, and a rotor 13, which isarranged between two poles 14 formed by the lamination pack 11 of thestator 10.

The rotor 13, in particular, is constituted by a cylindrical annularpermanent magnet 15 on which a plastic element 16 is overmolded, formingan inner shank 16 a and end flanges 16 b.

The rotor 13 therefore has, as a whole, a cylindrical shape with anaxial hole 17 in which a shaft 18 is inserted; said rotor 13 can rotatefreely about said shaft.

The shaft 18 is in turn connected to a supporting structure,conveniently generally designated by the reference numeral 19 andconstituted in this case in practice by two complementary shells 20 and21 which enclose the assembly constituted by the stator 10, the rotor 13and the shaft 18, in any case allowing the shaft 18 to protrude with anend 18 a to which a load to be turned, shown in dashed lines anddesignated by the reference numeral 22, is rigidly coupled.

Each one of the two shells 20 and 21 is internally provided, at theregion of the shaft 18, with a corresponding tang, designated by thereference numerals 23 and 24 respectively, inside which a bush isprovided, designated by the reference numerals 25 and 25 respectively,which rotatably supports a corresponding portion of the shaft 18.

As mentioned, one of the two shells, particularly the shell 21, has athrough hole 27 which allows the end 18 a of the shaft 18 to protrude.

According to the invention, the rotor 13 is arranged in a hermeticcasing, generally designated by the reference numeral 28, which isrigidly coupled to the shaft 18 and contains a liquid.

In particular, the hermetic casing 28 comprises a cup-shaped element 29,which is rigidly coupled to the shaft 18, and a disk-shaped plug 30which is engaged between the cup-shaped element 29 and the shaft 18,with which it forms a seal by means of respective O-rings 31 and 32(rings which provide a static seal, since the regions on which they actdo not move with respect to each other).

The plug 30 has at least one through hole 33 for introducing a presetamount of liquid inside the casing 28; said hole must conveniently beclosed after introducing said liquid.

As an alternative, the seal between the plug 30 and the cup-shapedelement 29 can be provided in other manners, such as heat-sealing,ultrasonic welding, etcetera.

The cup-shaped element 29 and the plug 30 substantially constitute amonolithic body and the assembly is rigidly coupled to the shaft 18.

The coupling to the shaft can occur equally in various manners, forexample by interference (cup-shaped element 29 and/or plug 30), directovermolding on the shaft 18 of the cup-shaped element 29 or of the plug30, hot assembly, etcetera.

As to the liquid, it is conveniently a viscous fluid and motion istransmitted between the rotor 13 and the hermetic casing 28 by viscousdrag produced by the internal stresses of the working fluid.

A motor with a viscous coupling between the rotor 13 and thecorresponding load 22 has thus been provided which ensures the startupof said motor in conditions which are fully similar to those of anasynchronous motor.

The introduction of a smooth variation between the rotation rate of therotor 13 (which is fixed in the synchronous motor) and the rotation rateof the load 22 (which is variable during the startup transient) allowsto start the motor until it reaches the steady-state rotation rate.

Geometric conditions being equal, transmission efficiency is a functionof the viscosity of the working fluid used.

According to the Reynolds-Petroff theory, there is an inverseproportionality relation between the gap between the rotor 13 and theinternal wall of the casing 28, particularly in a radial direction butalso in an axial direction, and accordingly said gap is convenientlygiven appropriate dimensions in order to achieve the highest efficiency.

This first embodiment of the invention is characterized by compactoverall size, simple construction, quiet startup, silent operation, lowconsumption and low cost.

Moreover, the rotor 13 is in a casing 28 which is fully hermetic and isthus insensitive to external aggressive agents.

As regards quietness in operation, the fact that the rotor 13 is coupledto the load 22 in a viscous manner entails damping of the nonlineartorque oscillations that are typical of the motion of a synchronousmotor.

This means less vibration and therefore quieter operation.

With reference now to FIG. 4, in a second embodiment there are again astator 110, a rotor 113 and a shaft 118; said shaft is connected to asupporting structure 119 provided with two shells 120 and 121.

The shaft 118 is, in this case too, coupled at one end 118 a to the load122 and is rotatably connected, by means of bushes 125 and 126, to tangs123 and 124 formed inside the shells 120 and 121.

The rotor 113 is again cylindrical and composed of a permanent magnet115 and of a plastic element 116 which is overmolded to form an internalshank 116 a and two end flanges 116 b, but a bladed impeller 135 extendsfrom the flange located towards the load 122 and acts as a pump for theliquid contained in the hermetic casing 128 in which the rotor 113 isarranged.

The impeller can of course extend from either of the two end flanges 116b as convenient.

The hermetic casing 128, in this case too, comprises a cup-shapedelement 129 and a plug 130 which forms a seal by means of O-rings 131and 132 and is provided with a closeable hole 133 for introducing theliquid.

According to the invention, a bladed impeller 136 extends from the partof the plug 130 that lies inside the cup-shaped element 129; saidimpeller is coaxial to the previous impeller 135 and faces it.

The impeller 136 constitutes a turbine which is rigidly coupled to theload 122 since it is integrated in the hermetic casing 128, which is inturn rigidly coupled to the shaft 118.

The casing 128, as in the preceding case, can be hot-fitted on the shaftor overmolded directly, or the cup-shaped element 129 and/or the plug130 can be fitted on the shaft 118 with an interference fit. The shaftis indicated in this embodiment by the reference numeral 218.

In this case too, the seal between the cup-shaped element 129 and theplug 130 can be provided alternatively by heat-sealing, ultrasonicwelding, etcetera.

Accordingly, a viscous coupling is formed in which the impeller 135(pump), turned by the rotor 113, supplies kinetic energy to the workingfluid contained in the casing 128.

The kinetic energy is converted into pressure energy (head) by the shapeof the vanes of the meridian ducts of the impeller 135.

The working fluid moved by the vane ducts of the impeller 135 starts tocirculate in the vane ducts of the impeller 136 (turbine), whichaccordingly starts to rotate.

Accordingly, drag occurs between the rotor 113 and the assemblyconstituted by the casing 128 and the shaft 118.

The load 122 is therefore turned.

The load turns in the same direction as the rotor 113.

A motor with a viscous coupling between the rotor 113 and thecorresponding load 122 has thus been provided which ensures the startupof said motor in conditions which are fully similar to those of anasynchronous motor.

The introduction of a smooth variation between the rotation rate of therotor 113 (which is fixed in the synchronous motor) and the rotationrate of the load 122 (which is variable during the startup transient) infact allows to start the latter until it reaches the steady-staterotation rate.

With respect to the previous embodiment of the motor, this embodimentdiffers in that it has a higher efficiency.

It can be convenient to use, for the pump and the turbine, two differentnumbers of vanes which are prime to each other, in order to prevent themeridian ducts of the pump from simultaneously facing those of theturbine during operation.

The motor with viscous coupling is characterized, as mentioned, by highefficiency, compact size, simple construction, quiet operationparticularly at startup, low consumption, low cost, and a rotor which iskept separated from any aggression of pollutants.

It should also be noted that the high efficiency of thisviscous-coupling motor allows applications also for transmitting loadswith low inertia when one wishes to decouple the load from the motor forany reason (for example to improve static torque, noise levels,etcetera).

With particular reference now to FIG. 5, a third embodiment of thesynchronous motor is similar to the preceding one and differs from it inthat the rotor, now designated by the reference numeral 213, is againcomposed of a permanent magnet 215 and of an overmolded element 216which forms an internal shank 216 a and end flanges 216 b but has twomutually opposite bladed impellers 235 a and 235 b which act as pumpsand protrude from both end flanges 216 b.

Likewise, the hermetic casing, now designated by the reference numeral228, is again composed of a cup-shaped element 229 and of a sealing plug230, but now it is internally provided with two bladed impellers whichact as turbines and are designated by the reference numerals 236 a and236 b respectively; one is located at the bottom of the cup-shapedelement 129 and one is arranged at the plug 230, so as to form twoviscous couplings with the impellers 235 a and 235 b.

This embodiment can also be provided when the torques to be transmittedare higher than in the preceding embodiment, maintaining the cylindricalstructure of the casing 228.

With reference now to FIGS. 6 to 8, in a fourth embodiment the rotor,now designated by the reference numeral 313, is again composed of apermanent magnet 315 and of an overmolded plastic element 316, which inturn forms an internal shank 316 a and end flanges 316 b. In this case,however, the rotor 313 is not cylindrical; rather, the end flange 316 barranged towards the load 322 is shaped so that it expands into an endportion which has a larger diameter and in which a set of vanes 335 b,similar to the preceding ones but larger, is formed. The shaft in thisembodiment is indicated by the reference numeral 318.

In this case too, the expansion can be provided equally, as convenient,on either of the end flanges.

The other end flange 316 a is identical to the preceding ones and thushas a set of vanes 335 a formed within the cylindrical bulk of this partof the rotor 313.

Likewise, the hermetic casing 328, again composed of a cup-shapedelement 329 and of a hermetic plug 330, has dimensions which aresuitable to follow the shape of the rotor 313 and therefore has anexpanded region in the portion where the cup-shaped element 329 and theplug 330 meet.

The plug 330 has a set of vanes 336 b which is suitable for the set ofvanes 335 b of the rotor 313, while at the opposite end the bottom ofthe cup-shaped element 329 has a set of vanes 336 a which is suitablefor the set of vanes of the corresponding head of the rotor 313.

This configuration is suitable to drive particularly high loads 322;accordingly, a size increase is provided for one of the viscouscouplings, particularly the coupling that technically allows suchenlargement, i.e., the coupling of the end of the casing 328 throughwhich the rotor 313 is inserted.

With reference now to FIGS. 9 to 12, in a fifth embodiment theconfiguration of the rotor, now designated by the reference numeral 413,and of the casing, now designated by the reference numeral 428, issimilar to the fourth embodiment, except that the bladed impeller 435 bwhich lies, in the case of the drawings, towards the load 422 is nolonger rigidly coupled to the rotor 413 but is coupled thereto by meansof a toothed traction coupling generally designated by the referencenumeral 437. The shaft in this embodiment is indicated by the referencenumeral 418.

The traction coupling comprises an axial eccentric tooth 438 whichprotrudes from the end flange 416 b of the element 416 that isovermolded on the permanent magnet 415 and a similar axial eccentrictooth 439 which protrudes from the corresponding facing region of theimpeller 435 b.

Accordingly, for a certain extent of a complete turn (advantageously 180sexagesimal degrees), the rotor 413 is uncoupled from the loadconstituted by the bladed impeller 435 b, which can start freely beforeturning said impeller.

The advantage is that static torque is reduced, motor startup isfacilitated also with considerable loads and efficiency is thusimproved.

As in the fourth embodiment, the casing 428 is provided with a bladedimpeller 436 b which faces the impeller 435 b and, on the opposite sideof the rotor, with an impeller 435 a and a corresponding impeller 436 aon the bottom of the casing 428.

In this case a shock-absorbing elastomeric element 440 (fitted on aprotruding tang 441 formed on the end flange 416 b) is also providedwhich is arranged between the teeth 438 and 439 indeed to cushion theirmutual impacts at startup.

With reference now to FIGS. 13 and 14, a sixth embodiment has a rotor,now designated by the reference numeral 513, and a casing, nowdesignated by the reference numeral 528 (with a cup-shaped element 529and a plug 530), which are identical to those of the preceding fifthembodiment and thus also have a toothed traction coupling which is nowdesignated by the reference numeral 537.

The impellers 535 a and 535 b, as well as the impellers 536 a and 536 b,again have different dimensions as in the preceding impellers.

In this embodiment, the supporting structure of the shaft 518 isconstituted by a container 519 which substantially matches the shape ofthe casing 528 that is contained therein and is accordingly constitutedby a cup-shaped element 520 and by a closure plug 521.

On the bottom of the cup-shaped element 520 there is provided a tang 523for a bush 525 that supports one end of the shaft 518, while on the plug521 a tang 524 is provided for a bush 526 for supporting the part of theshaft that is adjacent to the end that supports the load 522.

In this case, the supporting structure of the shaft 518 allows toassemble the rotor part separately from the stator part and producesfurther insulation between the stator and the rotor, which may benecessary in some applications.

The invention thus conceived is susceptible of numerous modificationsand variations, all of which are within the scope of the inventiveconcept.

All the details may also be replaced with other technically equivalentelements.

In practice, the materials employed, so long as they are compatible withthe contingent use, as well as the dimensions, may be any according torequirements.

What is claimed is:
 1. An electric motor with permanent-magnet rotorcomprising a stator, with an electromagnet constituted by a laminationpack and associated windings, and a rotor, which is arranged between twopoles formed by the stator and is axially crossed by a shaft which isrotatably connected to a supporting structure, wherein said rotor ismounted freely on the rotation shaft to which the load is applied and iscontained in a hermetic casing which is rigidly coupled to said shaftand contains a working fluid, said rotor and said hermetic casing beingshaped so as to mutually interact only by means of the working fluid,thus allowing smooth variations between the speed of the rotor and thespeed of the casing and accordingly between the rotor and the appliedload.
 2. An electric motor with permanent-magnet rotor according toclaim 1, wherein a gap is provided between the outer surface of saidrotor and the inner surface of said casing in which said rotor isaccommodated.
 3. An electric motor with permanent-magnet rotor accordingto claim 1, wherein a bladed impeller is rigidly coupled to at least oneof the ends of said rotor and interacts with a corresponding bladedimpeller which is rigidly coupled to said casing and is arrangedfrontally thereto.
 4. An electric motor with permanent-magnet rotoraccording to claim 1, wherein a bladed impeller is coupled, by means ofa toothed traction coupling, to one of the ends of the rotor andinteracts with a corresponding bladed impeller which is rigidly coupledto said casing and is arranged frontally thereto.
 5. An electric motorwith permanent-magnet rotor according to claim 1, wherein said rotorcomprises a cylindrical annular permanent magnet on which a plasticelement is overmolded, said plastic element forming an internal tang andend flanges, said rotor thus having, as a whole, the shape of a cylinderwith an axial hole in which said shaft is inserted, said rotor beingable to rotate freely on said shaft.
 6. An electric motor withpermanent-magnet rotor according to claim 1, wherein said hermeticcasing comprises a cup-shaped element which is rigidly coupled to saidshaft and a plug which is inserted between said cup-shaped element andsaid shaft, with which it forms a seal.
 7. An electric motor withpermanent-magnet rotor according to claim 6, wherein said casing has atleast one closeable through hole for introducing a preset amount of theliquid inside it.
 8. An electric motor with permanent-magnet rotoraccording to claim 6, wherein said seal between the plug and thecup-shaped element is provided by means of O-rings, heat-sealing, orultrasonic welding.
 9. An electric motor with permanent-magnet rotoraccording to claim 6, wherein the connection between said casing andsaid shaft occurs by interference fit, direct overmolding on the shaftof the cup-shaped element or of the plug, or hot assembly.
 10. Anelectric motor with permanent-magnet rotor according to claim 3, whereina set of vanes connected to said rotor and a set of vanes connected tosaid casing have two different numbers of vanes which are prime to eachother.
 11. An electric motor with permanent-magnet rotor according toclaim 4, wherein said traction coupling comprises an axial eccentrictooth which protrudes from said end flange of the element that isovermolded on said permanent magnet and a similar axial eccentric toothwhich protrudes from the corresponding facing region of said impeller,which is thus connected to said rotor.
 12. An electric motor withpermanent-magnet rotor according to claim 11, wherein a shock-absorbingelastomeric element is arranged between said teeth in order to cushiontheir mutual impacts at startup.
 13. An electric motor withpermanent-magnet rotor according to claim 6, wherein said rotor has twofirst mutually opposite bladed impellers which extend from both headsand said hermetic casing is internally provided with two second bladedimpellers, one at the bottom of said cup-shaped element and one at saidplug.
 14. An electric motor with permanent-magnet rotor according toclaim 13, wherein said rotor has, equally on either of its two ends, aset of vanes which is larger and, at the other end, a set of vanesformed within the cylindrical volume of that part of the rotor.
 15. Anelectric motor with permanent-magnet rotor according to claim 14,wherein said hermetic casing has dimensions which are suitable to matchthe shape of said rotor and thus has an enlarged portion in the regionwhere said cup-shaped element and said plug meet, said plug beingprovided with a set of vanes which is suitable for the set of vanes ofsaid rotor; the bottom of the cup-shaped element having, at the oppositeend, a set of vanes which is suitable for the set of vanes of thecorresponding end flange of said rotor.
 16. An electric motor withpermanent-magnet rotor according to claim 1, wherein said supportingstructure for said shaft is constituted by two complementary shellswhich enclose the assembly constituted by the stator, the rotor and theshaft, allowing said shaft to protrude with an end to which the load isconnected.
 17. An electric motor with permanent-magnet rotor accordingto claim 1, wherein said supporting structure comprises a containerwhich substantially matches the configuration of the casing containedtherein and is accordingly constituted by an additional cup-shapedelement and by an additional closure plug through which said shaftpasses.
 18. An electric motor with permanent-magnet rotor according toclaim 17, wherein each one of said two elements of said container isinternally provided, at the region of said shaft, with a correspondingtang which is internally provided with a bush that rotatably supports acorresponding portion of said shaft.